The present invention relates to industrial I/O controller circuits for industrial control systems, and in particular to industrial I/O controller circuits for frequency input modules that measure system frequencies using adaptive threshold voltage and/or adaptive hysteresis feedback.
Industrial controllers are specialized computer systems used for the control of industrial processes or machinery, for example, in a factory environment. Industrial controllers typically comprise I/O controller circuits to accomplish different functions as part of the industrial control system. One such function is measuring the frequency or revolutions per minute (RPM) of a moving machine, such as a large rotating toothed wheel, so that some action may be taken by the industrial control system. For example, an industrial controller may comprise an I/O controller circuit used for measuring the frequency of a turbine engine so that additional power can be applied to (or removed from) the turbine engine by the industrial controller based on the measured frequency.
In such frequency measuring applications, the electromagnetic field produced by the physical motion, of the moving machine may be sensed by a sensor positioned in close proximity to the machine. The sensor may then, produce, an electrical signal of varying frequency and amplitude, approximately corresponding to the periodic motion of the machine. A variable reluctance sensor comprising a permanent magnet and a pick up coil is typical sensor that may be used for such motion sensing applications.
The electrical signal may be inputted to a threshold detector and compared to a predetermined threshold level to produce a square wave digital output signal having first and second states, e.g. logic zero and logic one. The states of the square wave digital output may reflect the frequency of the machine. The output may subsequently be processed by digital hardware, software or any combination thereof in the industrial control system. In operation, each time the electrical input signal crosses the predetermined threshold level, the threshold detector toggles the square wave digital output signal it produces between the first and second states.
In application, the electrical signals produced by sensors are often subject to noise and distortion due to electromagnetic interference (EMI). EMI may originate from other machinery in the factory environment, or by cross-coupling from neighboring channels, such as by another electrical signal from a nearby sensor. The result of EMI is a loss of signal integrity in the electrical signal, which may cause random noise and ringing to the electrical signal.
To oppose the effects of EMI, ferrite beads or other filtering circuitry may be applied to the electrical signal to improve signal integrity. Such filtering inherently reduces the time resolution of the derived digital signal by removing high-frequency components of the sensed signal. The use of hysteresis may also be employed to reduce the effects of electrical noise.
The strength of the electrical signal from a variable reluctance type sensor may vary significantly depending on the placement of the sensor and speed of movement of the sensed metal element. For this reason, the threshold used to produce the desired square wave signal can be a complicated exercise. Placing the threshold too low will make the sensor susceptible to electrical noise whereas placing the threshold too high may cause the system to fail to detect low level signals from the variable reluctance sensor at low machine speeds. Similar problems arise with respect to determining the amount of hysteresis that is optimum.